Field
Appropriate signal processing may be beneficial in a variety of communication systems and elements thereof. For example, time domain digital pre-distortion may benefit from suitable treatment of frequency switching.
Description of the Related Art
Digital pre-distortion can be used within linearization of amplifiers, such as radio frequency (RF) power amplifiers. Typical conventional linearization algorithms manipulate the baseband signal at time-domain within linearization loop so that the RF power amplifier output is adaptively forced to be linear. Many time-domain algorithms can perform well for narrow band signals but performance drops quickly when the signal bandwidth is increased.
The reason for such drop in performance may be the nature of the RF power amplifier (PA) distortion. PA distortion can be linearized through circuit level design and through transistor technology development/selection. Digital pre-distortion (DPD) can be considered a system level linearizer that can further improve the RF PA linearity.
From a digital pre-distortion point of view the distortion can be viewed as nonlinear variation of complex gain. An example of a DPD therefore can be a non-linear gain pre-distorter. A linear baseband signal can be multiplied with a non-linear gain element at the digital time domain. Hence, the original baseband signal can be distorted. The result can be referred to as a pre-distortion signal. An adaptive algorithm can adjust the nonlinear gain element so that when the pre-distortion signal goes through an RF power amplifier the PA output may be linear.
When the base band signal bandwidth is increased, the RF power amplifier distortion mechanisms can change. Thus, conventional DPD linearization performance can drop. The performance may drop because the distortion starts to have frequency dependent magnitude changes. These changes may not be seen when the signal bandwidth is narrow. The frequency dependent distortion magnitude change can be referred to as a memory effect. Memory effects that are asymmetric are conventionally difficult to linearize.
Two-tone power and tone-spacing sweeps can be used to get first hand predictions of the PA memory effects. The amplifier can be excited with a two-tone signal and the tone spacing can be varied while the input signal power is kept constant. Because the input signal power is constant, the distortion should not change while the tone-spacing changes. However, intermodulation (IM) distortion (IMD) levels may change because the distortion mechanisms change as function of signal bandwidth, in addition to other contributions. Thus, the lower and upper IMD components may change differently as function of signal bandwidth, two-tone spacing.
Thus, the distortion, or non-linear variation of the gain, can have memory and asymmetry. The two-tone test may give an indication of RF power amplifier broadband linearity capabilities and may give first-hand information for DPD point of view.
On the other hand, a memoryless non-linear gain pre-distorter may be unable to produce a pre-distortion signal that changes as function of bandwidth. Some non-linear gain algorithms may contain filters for different order distortion voltages so that there is wider set of distortion contributors that can be adaptively controlled. Still, the asymmetry may be difficult to linearize conventionally.
FIG. 1 illustrates generation of distortion within an RF power amplifier. As shown in FIG. 1, there can be long and short time constants within distortion and distortion may be asymmetric against a center frequency. The two-tone test does not completely predict RF PA linearity performance. For example, distortion may be different in the case of a modulated signal.
FIG. 1 visualizes the distortion generation within RF amplifier in general. The RF power amplifier resistive and capacitive nonlinearities can create nth-order distortion currents. These distortion currents can transform into circuit nodal voltages through the impedances or trans-impedances seen by the individual distortion current contributor. The non-linear currents can be controlled by broadband voltage spectrum. In-band distortion can also be contributed by the out-of band nodal voltages through up-conversion and down-conversion process of the non-linear elements, which can also be referred to as self-mixing.
Hence, the RF power amplifier linear gain may be varied. The gain variation can be non-linear and the nonlinearity can contain short and long time constant filtering mechanisms of the circuit. Bias circuits and thermal constants can introduce long-time constant memory effects. On the other hand, in-band and higher frequency nodal impedances can introduce short time constant memory effects. These effects may add complexity to the RF power amplifier base band modelling, for example, to the pre-distortion modelling.